The invention relates to a method and a device for inspecting transparent sheets or webs for the presence of defects, particularly enclosed core seeds, where the web to be tested is scanned by a flying light spot. The light passing through the web is directed onto a diffuser plate which has transparent and opaque regions and behind which a receiver is disposed. The photoelectric transducer disposed in the receiver feeds pulses which correspond to the intensity of the light impinging on the diffuser plate to a computer.
Transparent webs in accordance with the present invention are continuously manufactured webs made of a transparent plastic material or glass. Since the formation of core seeds is a problem that occurs particularly with float glass lines, the invention is explained with reference to the inspection of float glass webs. It is, however, not limited thereto.
Inspection devices for glass webs are known from DE-OS 31 29 808 and DE-OS 32 23 215. It is common to both publications that the material web to be tested is scanned with a flying light spot and that light which is deflected into the glass by a core seed is detected at the edge of the material and evaluated. If the core seed is located in the center of the material to be tested, the distance to be covered by the light across the glass is very long, and the absorption is consequently very high. Particularly during the inspection of slightly colored glass, this absorption can be so high that no evaluable light signals can be received at the edge of the web. Another disadvantage of this known device is that the receiver at the edge of the material web must be relatively well insulated against incident foreign light in order to detect and evaluate, via the photoelectric transducer, the minor intensities of the light deflected at the core seeds. Moreover, deformation occurring at the margin and deflecting the light must be taken into account which requires additional devices to collect the light.
It is another disadvantage that the width must be continuously monitored in order to avoid damages of the laterally disposed receivers.
It is an object of the present invention to improve these known devices such that even more intensely colored glass or glass webs of a large width can be properly inspected for core seeds.
In accordance with the invention, this object is achieved with a method for inspecting transparent webs for the presence of defects, particularly enclosed core seeds. The transparent web to be tested is scanned with a flying light spot and the light passing through the web is directed onto a receiver system which has transparent and opaque areas and behind which a receiver is disposed. The photoelectric transducer disposed inside the receiver feeds pulses to a computer which correspond to the light intensities incident to the receiver system. The accomplishment is characterized in that the light spot is directed such that when the material web is free of defects, it covers the opaque and the transparent area of the receiver system in a continuous alternation, the change of area and the deviations of the light intensities caused by defects are supplied to the computer as pulses. These pulses are then evaluated while separated according to the areas of the receiver system.
The problems that occur during the inspection of glass webs are such that a great number of different defects can occur, however, in addition to these error signals, it is also possible that like signals occur which may be due to a contamination of the surface of the glass web and hence also lead to error messages, so-called false defects. Different inspection methods were developed corresponding to the different ways of inspecting. A distinction is made between measurements in reflection and transmission. They can further be divided in the so-called mirror reflection, hence inspection in the bright field, hereinafter referred to as R/S measurement. Further, the so-called diffuse reflection, hence the inspection in the dark field, referred to as R/DF. Moreover, there is direct transmission, also a measurement in the bright field, referred to as T/D. The is direct transmission in the close dark field, referred to as T/DA and diffuse transmission in the dark field, referred to as T/DF.
In R/S the directly reflected light is detected via a photoelectric transducer. Light fluctuations then occur when a non-reflecting defect or an impurity does not allow the total amount of light to be reflected. In R/DF, the receiver is disposed such that only diffusely reflected light can be received. The receiver is in both cases disposed above glass web to be inspected.
Other possibilities of inspecting transmission include a receiver which is disposed underneath the glass web to be tested. When measuring in direct transmission T/D, the light beam which has passed the glass web enters a light-open gap and is deflected out of this gap--and possibly darkened by impurities. The deflection by defects and the darkening by impurities prevents the light beam from entering the receiver as a complete beam and the occurring light intensity is reduced and, hence, the photoelectric transducer triggers a pulse. When measuring in the close dark field T/DA, the light beam also passed through the glass web to be inspected. However, in case there are no defects, it does not reach the receiver but is blocked by an opaque layer, the so called stopper, which is applied onto the receiver disk. Only if the beam is deflected by a defect, it can enter the receiver at the right or left side of the stopper and then generate a pulse signal in the photoelectric transducer. In an inspection according to T/DF, the one or several receivers are disposed outside the direct beam passage such that only diffuse light enters the one or several receivers if the light beam encounters a defect while scanning.
Depending on the inspection method selected, external influences may have a more or less negative effect. The deposition of dirt, for example, on or under the glass web to be tested greatly affects the bright field, i.e. the evaluation during the inspection according to R/S and T/D is greatly impaired. During the inspection in T/DA, however, the evaluation is significantly less affected.
The R/S method hence permits detecting major deformation defects in the material web and identifying reflecting defects. However, it is not possible to detect small defects from small deformations or to spot core seeds.
Method T/DA permits both the detection of coarse surface deformations and surface deformations that are small in dimension. However, it is not possible to recognize small core seeds nor to identify reflecting defects. Another false defect is caused by so called plating which refers to the vertical and twisting movement of the moving glass web. This also greatly impairs mirror reflection inspection, hence R/S.
Method T/D permits recognizing coarse deformations and detecting core seeds. It is, however, not suitable for the identification of reflecting defects. The detection of small defects with deformations is not as sensitive as in T/DA. Prior art hence offered possibilities of successively arranging several inspection stations in order to achieve a complete inspection of a glass web. This, however, calls for the installation of three complex aggregates, one behind the other which is not possible due to the limited space in the production lines. Moreover, high costs also oppose this.